# Physics



## [O] (Oct 7, 2010)

So.... I am not doing so well in my Physics class. We're on circular motion and centripetal acceleration right now. So, TCoD, would you know how to do the following problem?

A penny of mass 3.1 g rests on a small 21.1 g block supported by a spinning disk of radius 23 cm. The coefficients of friction between the block and the disk are 0.798 (static) and 0.64 (kinetic) while those for the penny and block are 0.674 (static) and 0.45 (kinetic.) What is the maximum speed of the disk without the block and penny sliding? The acceleration of gravity is 9.8 m/s^2. Answer in units of rpm.

I honestly had no idea how to approach this problem other than frictional force being μmg.


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## Music Dragon (Oct 7, 2010)

Hurr hurr! The block and penny are moving circularly, which means there's a centripetal force somewhere. In this case, that force is friction.

Does that help?


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## Tarvos (Oct 7, 2010)

Approach the problem step by step. I do not want to work it out for you as you will learn zero problem solving skills from it, but I can do this for you:

1) Ask yourself, what variable do they want to know? In this case, they want a rotational frequency.

2) What is given? Frictional coefficients (this suggests you need to look up equations that describe frictional force). Gravitational acceleration . What you need to do is find all the possible relations you could use, summarise them, write them down, and see how you can combine variables to solve the puzzle.

I'll give you another few hints: Your mass is at rest. This means resultant forces are zero. You can set up a force balance over both the penny and the disk to solve the problem (most likely both will be equal to each other or some shit).

Centripetal force (centrifugal force does not exist) doesn't exist solo - there is always something external causing it. It can be gravity; it can be electrical forces; in this case, it is friction, as MD suggests.

This means you should have enough data to solve the problem. I'll let you do the math yourself.

Remember
1) what is asked for?
2) what is given?
3) what is my body/control volume, what are forces operating on? Make a sketch unless a drawing is given
4) which equations relate given variables to asked variables? Do I need to calculate a different variable before I can get to my final answer? In that case, how is the middle variable related to the knowns?
5) Work in symbols. Do the arithmetic later.
6) Once you've figured out what equations give you your final answer, do the arithmetic.
7) Check if your answer makes sense. If your rpm is something like 10000000, you've probably done something wrong.


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